Watermarking of video signals is, generally, the inclusion within the video itself of additional information. This can be useful to provide an embedded identification of the source of a video, to keep track of where and for how long a video is played, and to communicate information via the video to an ancillary device. Prior art techniques for watermarking video signals typically encoded the additional information in an analog format within the video itself using the luminance of the video to carry the additional information. However, the human visual system is very sensitive to the luminance signal, and so a person viewing a watermarked signal easily perceives distortion which is caused by the changes made to the video signal to convey the additional information when there is an attempt to increase the bit rate of the additional information beyond a certain point, e.g., beyond 120 bits per second. Thus, although the prior art's techniques of watermarking of video signals has had some success in certain applications, such success has been limited by the extremely small bit rate that is achievable without perceivable distortion by a person viewing the video signal carrying the additional information.
In previously filed U.S. patent application Ser. No. 10/342704, which is incorporated by reference as if set forth fully herein, I, along with my coinventor, recognized that the human visual system is much less sensitive to chrominance than to luminance. Therefore, we developed a system for digital watermarking a video signal that inserts the additional information of the watermarking signal on the chrominance component of the video signal rather than on its luminance signal. Thus, the additional information is “impressed” upon the chrominance component of the video signal. Advantageously, although there may be significant distortion of the chrominance component, especially when the additional information has higher bit rates than is achievable without perceivable distortion by the prior art, nevertheless such distortion will not be detected by the human visual system, provided it is appropriately managed. Thus, the additional information can have a higher bit rate as compared with that achievable by the prior art, e.g., bit rates greater than 150 bits per second can be achieved. Further advantageously, the additional data can be recovered from the video signal even after the video signal watermarked with the additional data is compressed using the Motion Picture Expert Group (MPEG)-1 and MPEG-2 encoding systems.
The particular chrominance portion selected to carry the watermarking for any pixel is selected in U.S. patent application Ser. No. 10/342704 by a color selection unit. The color selection unit determines the selected chrominance component as a function of the RGB and the YUV representations of the pixel using a prescribed formula. Since digital video is often transmitted only in YUV format, to use the system of U.S. patent application Ser. No. 10/342704 with such YUV formatted video, it is necessary to develop therefrom the corresponding RGB formatted video. Disadvantageously, to do so requires considerable processing power. Furthermore, although it is very good, it was later discovered that the mathematical model underlying the formula employed in the selection process of U.S. patent application Ser. No. 10/3427 does not necessarily always produce flicker-free results.